1. Field of the Invention
The invention relates to a spread spectrum communication apparatus having a transmission power control function.
2. Related Background Art
A radio communication system is influenced by a fading which occurs in a space or an interference from another radio communication system and has a possibility such that the self radio communication state deteriorates.
Therefore, when a propagating state of a space, namely, a line state is good, a transmission power of the self transmission unit is reduced to thereby decrease an interference on another system. When the line state is bad, by increasing the transmission power, the interference on another system is relatively decreased, thereby coping with the fading.
FIG. 2 shows such an example.
In FIG. 2, reference numeral 1 denotes a radio apparatus; 2 a transmission unit; 3 a reception unit; 4 a duplexer; 5 and 7 antennas which are commonly used for both of the transmission and reception; 6 a radio apparatus; 8' a duplexer; 9 a reception unit; 10 a transmission unit; 11 a reception level detection processing circuit for detecting a reception level in the reception unit 9 and transmitting to the radio apparatus 1; and 12 a transmission power control circuit for receiving reception level information from the reception level detection processing circuit 11 of the radio apparatus 6 and controlling the transmission power of the transmission unit 2 of the radio apparatus 1.
A construction of the reception unit 9 in FIG. 2 will be further described in more detail. In FIG. 3, reference numeral 13 denotes, for example, an LNA (low noise amplifier); 14 a frequency converter; 15 a local oscillator; 16 a BPF (band pass filter); 17 an IF (intermediate frequency) amplifier; and 18 a detection circuit.
The received signal is amplified by the low noise amplifier 13 and is frequency converted by the local oscillator 15 and frequency converter 14 and is amplified to a predetermined signal by the IF amplifier 17 through the BPF 16.
The amplified signal is supplied to a demodulation unit (not shown) and is converted to a DC voltage according to the received signal level by the detection circuit 18 and is supplied to the reception level detection processing circuit 11.
In the conventional apparatus mentioned above, however, since the received signal is detected by using the IF (intermediate frequency) signal, in a system such as a radio LAN system such that a plurality of users commonly share the same frequency band and are identified by codes, an inconvenience such that the signals of the users other than the user who is at present communicating are also detected occurs.
There is a case where the signal cannot be detected when a noise level of the use band is increased or the like by the signals which are generated from the other transmitters in the same band.
FIG. 4 shows a construction of a conventional spread spectrum communication apparatus disclosed in U.S. Pat. No. 4,958,359.
Source data which is digital data, is spread-modulated by a spread modulator 65 by using a predetermined spread code generated by a code generator 68. The output of the spread modulator 65 is multiplied by a mixer 64 with a signal of a predetermined frequency generated by a local oscillator 67. Accordingly, the output of the mixer 64 has a predetermined intermediate frequency and it is amplified by a variable gain amplifier 63. The output of the variable gain amplifier 63 is frequency-converted by a mixer 62 and a local oscillator 66.
The output of the mixer 62 is supplied to a filter 61 where only a required band component is extracted and it is transmitted from an antenna 51 through a common unit 52.
In a receiver, a received signal from the common unit 52 is supplied to a mixer 53. Another input to the mixer 53 is a signal mixed by a mixer 58, which receives a predetermined code generated by a spread code generator 57 and a signal generated by a local oscillator 60. The local oscillator 60 generates a signal of a frequency which is shifted from a receiving frequency (frequency of carrier of a desired signal) by the intermediate frequency.
If the codes used in the transmitter and receiver for modulation and demodulation are phase-synchronized, an intermediate signal having a band width corresponding to the source data is produced at the output of the mixer 53. The output of the mixer 53 is supplied to an intermediate frequency filter 55 which has a band width corresponding to the source data. An intermediate frequency signal produced by the intermediate frequency filter 55 is amplified by a variable gain amplifier 54 and the output of the variable gain amplifier 54 is applied to a demodulator 56 which demodulates the source data.
The variable gain amplifier 54 voltage-controls its gain by an AGC (automatic gain control) voltage generated by an AGC voltage generator 69, which receives the output of the variable gain amplifier 54 and controls the amplifier 54 to keep a level thereof constant.
A gain of the variable gain amplifier 63 of the transmitter is also controlled by the AGC voltage generated by the AGC voltage generator 69 provided in the receiver.
In an initial state, that is, when communication between stations has not been established, the received signal is spread by the code generated by the code generator 57 of the receiver. Accordingly, the output of the intermediate frequency filter 55 is small and the AGC voltage generator 69 controls to increase the gains of the variable gain amplifiers 54 and 63.
If a signal from other station corresponding to the receiving channel, that is, a signal corresponding to the code generated by the receiver code generator 57 is applied to the antenna 51, the output of the intermediate frequency filter 55 increases after the phase synchronization of the codes for the modulation/demodulation has been established. As a result, the output signal of the amplifier 54 increases and the AGC voltage generator 69 controls the gain of the variable gain amplifier 54 so that the output of the variable gain amplifier 54 is kept at an appropriate level. The AGC voltage also controls the transmitter variable gain amplifier 63. The AGC voltage generator 69 determines a distance to other station by the level of the received signal. If it determines that the distance is short, it lowers the level of the transmitting signal in accordance with the distance. For a near station, the transmitting signal level is low, and for a distant station, the transmitting signal level is high. Accordingly, the electric field strength at the antenna of the other station is of proper level.
On the other hand, when the signal is to be sent from the present station to, the gain of the variable gain amplifier 63 is initially controlled to a high level as described above. Accordingly, the electric field strength at the antenna of the other station is unduely high. However, as the other station responds, the input signal to the AGC voltage generator 69 increases. As a result, the AGC voltage generator 69 controls the gain of the variable gain amplifier 63 such that the electric field strength at the antenna of the other station is at the proper level.
However, in the apparatus of FIG. 4, the transmission power cannot properly be controlled before the synchronization is accomplished between the transmission and the reception.